Having an IP address in the connection notice simply means their IP addresses could not be resolved to a host. This is not a good way to tell which users are bots and which are not.

To better tell which users are bots and which are not, you should look at messages they may be sending to channels (both messages to the channels, and part messages), look at quit messages, and also look for any channels they are crowding in.

If they are all in 1 channel, simply stay in that channel and gline anyone who joins that is not human. There might also be commands you can send to the bots though the topic or messages that can make them remove themselves.

Also BE SURE you are running a BOPM or other bad people monitoring program as these would cut down on how many bots join. You can get instructions for setting up a BOPM here: http://unreal.x-tab.org/faq#InstallBOPM

A BOPM is not a perfect solution, and there will always be some that get through. The only perfect solution would be to not run an IRCd and use another networks services to fill your needs (let them worry about it).

Again, you should look for a control pattern or other patterns in messages and realnames. All botnets have some kind of pattern or control channel, even the ones used specifically for flooding and are 'random'.